Analysis of multiquantal transmitter release from single cultured corticalneuron terminals

Application of single synapse recording methods indicates that the amplitud e of postsynaptic responses of single CNS synapses can vary greatly among r epeated stimuli. To determine whether this observation could be attributed to synapses releasing a variable number of transmitter quanta, we assessed the prevalence of multiquantal transmitter release in primary cultures of c ortical neurons with the action potential (AP)-dependent presynaptic turnov er of the styryl dye FM1-43 (Betz and Bewick 1992. 1993: Betz et al. 1996). It was assumed that if a high proportion of vesicles within a terminal wer e loaded with FM1-43 thr amount of dye released per stimulus would be propo rtional to the number of quanta released and/or the probability of release at a terminal. To rule out differences in the amount of release (between te rminals) caused by release probability or incomplete lending of terminals, conditions were chosen to maximize both release probability and terminal lo ading. Three-dimensional reconstruction of terminals was employed to ensure that bouton fluorescence was accurately measured. Analysis of the relation ship between the loading of terminals and release indicated that presumed l arger terminals (>FM1-43 uptake) release a greater amount of dye per stimul us than smaller terminals. suggesting multiquantal release. The distributio n of release amounts across terminals was significantly skewed toward highe r values, with 13-17% of synaptic terminals apparently releasing multiple q uanta per AP, In conclusion, our data suggest that most synaptic terminals release a relatively constant amount of transmitter per stimulus; however, a subset of terminals releases amounts of FM1-43 that are greater than that expected from a unimodal release process.